I read somewhere that whenever someone upload a project to Github, they should always include a readme file, for legal reasons.

What should this readme file say? I dont know much about laws. My project is 100% free and open-source. I use Slick2d(along with LWJGL) and TinySound, the rest have I code myself.

Should I be worried that some douche may sue me?

You include the author, date released, the copyright information (in your case, it would be open source so there isn't really anything to say here, just say so). Give credits where it is owned, might be nice to say what you use, for me I would thank the LibGDX creator, Box2D creator and any free art I use.

Also include a small description.

"This code works flawlessly first time and exactly how I wanted it"Said no programmer ever

A readme on Github is a bit different than a regular legal or instructional notice, at least IMO. It usually includes those things, though.

The readme is the 'front page' of your project on Github, such as here, beginning with the libGDX logo. It gives general information for your project, and links to get more detailed info, such as a project wiki.

I read somewhere that whenever someone upload a project to Github, they should always include a readme file, for legal reasons.

That would be a license, not a readme. GitHub actually provides you a selection of licenses you can choose when you create your repository.

Since you said your project is 100% free and open-source, you can either consider GPL licenses, the Apache License, or the Creative Commons license.

GPL licenses require you to put the licenses everywhere if you continue with the project.Apache License is what most of us GitHub project creators use.Creative Commons is like what the Wikipedia does, sharing information without saying who the author(s) is.

java-gaming.org is not responsible for the content posted by its members, including references to external websites,
and other references that may or may not have a relation with our primarily
gaming and game production oriented community.
inquiries and complaints can be sent via email to the info‑account of the
company managing the website of java‑gaming.org